Electron microscope employing a modulated scanning beam and a phase sensitive detector to improve the signal to noise ratio



Oct. 20, 1970 CHUSUKE MUNAKATA 3,535,516 ELECTRON MICROSCOPE EMPLOYING A MODULATED SCANNING BEAM AND A PHASE SENSITIVE DETECTOR TO IMPROVE THE SIGNAL TO NOISE RATIO Filed Oct. 6, 1967 2 SheetsShee-t l sujfce an E2 cam-R U U U J SOURCE INVENT OR BY Ja m- ATTORNEYS Oct. 20, 1970 CHUSUKE MUNAKATA 3,535,515

ELECTRON MICROSCOPE EMPLOYING A MODULATED SCANNING BEAM AND A PHASE SENSITIVE DETECTOR TO IMPROVE THE SIGNAL TO NOISE RATIO Filed Oct 6, 1967 2 Sheets-Sheet 2 case/LL0- VOLT- LAW? 3 snaps METER I 25- INTE- sms 3 vamas /5 O I SOURCE 25 INVENTOR ATTORNEY United States Patent US. Cl. 25049.5 Claims ABSTRACT OF THE DISCLOSURE A scanning type electron microscope wherein a specimen is scanned with an electron beam whose intensity is periodically varied and only an AC. component is detected from an electric signal originated in the specimen by the scanning with the electron beam.

This invention relates to scanning electron microscopes and more particularly to a novel and improved scanning type electron microscope which is so constructed that a specimen is scanned with an electron beam whose intensity is periodically varied.

As is commonly known, a scanning type electron microscope gives a visual indication of a scanning image of a specimen by scanning the specimen with an electron beam and detecting back-scattered electrons or secondary electrons reflected or emitted from the specimen, or by detecting a specimen current, or'by detecting a voltage generated in the specimen due to the so-called electron voltaic effect as disclosed, for example, in a paper appearing on pp. 929-936 of Journal of Electrochemical Society, 111, 1964, by T. E. Everhart, O. C. Welks and R. K. Matta. The prior art scanning type electron microscope has been accompanied by various unavoidable defects in view of the fact that the intensity of the scanning electron beam is set at a constant value which does not vary with time and therefore a signal detected from a specimen always takes the form of a direct current.

A typical structure of the prior art electron microscopes of the scanning type will be described With reference to FIG. 1 so that the invention can more clearly be understood. The prior art scanning type electron microscope shown in FIG. 1 includes an electron gun formed by an electron beam emitting filament 1, a Wehnelt cylinder 2, and an anode 3. The electron microscope further comprises an electron beam condenser lens 4 and an electron beam deflecting device 5 for deflectingly directing an electron beam 16 to a specimen 6, a detector 7 such as a photo-multiplier or a secondary-electron multiplier for detecting back-scattered or reflected electrons and secondary electrons, a DC. amplifier 8, on-oif switches 9, 10 and 14, a cathode-ray oscilloscope 11, a deflecting voltage source 12 for the deflecting device 5 and the oscilloscope 11, and a high voltage source 13 for the electron beam acceleration.

Suppose now that the specimen 6 to be scanned by the electron beam 16 is a semiconductor device having therein a p-n junction. Since, in such a case, the voltage generated in the specimen 6 due to the electron voltaic effect varies depending on the bias voltage applied to the specimen 6 from a bias voltage source 15, detailed information as to the structure of the p-n junction can be successfully obtained if it is possible to observe the manner of variation of the scanning image appearing on the cathode-ray oscilloscope 11 under a state in which the bias voltage is continuously applied to the specimen 6. However, in view of the fact that the electron beam 16 in the prior art electron microscope of the scanning type has generally a constant intensity which does not vary with time as described above, the voltage generated in the specimen 6 due to the electron voltaic effect appears in the form of a D.C. voltage and cannot be distinguished from the DC. bias voltage applied to the junction. It is therefore diflicult to distinguishably detect the voltage generated in the specimen 6 due to the electron voltaic effect. In order to remedy the above difficulty, an attempt was previously made to superimpose a DC. voltage on the voltage obtained from the specimen in such a manner as to cancel out the component of the bias voltage for thereby solely deriving the voltage generated due to the electron voltaic effect. However, it can be readily imagined that the proper regulation to cancel out solely the bias voltage component while continuously applying the bias voltage to the specimen is quite difficult to attain from a technical point of view.

Since further the voltage induced due to the electron voltaic effect is of the order of l()' volt, an attempt to amplify this voltage by use of the prior wide-band D.C. amplifier 8 would in many cases give a voltage level which is lower than the noise level of the amplifier 8, and it is thus impossible to effectively detect the induced voltage. A further difficulty arising from the above amplification stage is that a thermo-electromotive force of the order of 10- volt is simultaneously generated in most cases. In such a situation, it is impossible to distinguish whether the detected signal voltage represents the above-described induced voltage or Whether the signal voltage represents the thermo-electromotive force even if the noise level of the DC. amplifier 8 is lower than 10- volt.

Suppose then a case in which the detector 7 is in the form of a photomultiplier so that the back-scattered or reflected electrons and secondary electrons arriving from the specimen 6 can thereby be detected to obtain a scanning image of the specimen 6. In such a case, it is necessary to accelerate the back-scattered or reflected electrons or secondary electrons, to cause these electrons to impinge against a fluorescent substance (not shown) for making the fluorescent substance radiant, and to convert the light output emitted from the fluorescent substance into an electrical signal by means of the' photomultiplier. In this case too, the light signal derived by the radiation of the fluorescent substance has a constant intensity which does not vary with time, and admission of the slightest portion of external light into the photomultiplier would result in the impossibility of effective signal detection. Recently, a p-n junction is frequently employed in lieu of the photomultiplier, but the situation is still the same as in the case of the signal detection by the photomultiplier. The prior method of detecting the electron current flowing into the specimen 6 is likewise unsatisfactory because it is hardly possible to expect a high sensitivity in view of such a high noise level of the amplifier as pointed out previously. Thus, an effort to improve the detection sensitivity in the prior art arrangement inevitably results in the requirement for an increase in the electron beam current, which necessarily gives rise to a larger diameter of the electron beam and a reduction in resolution.

It is therefore an object of the present invention to provide a novel and improved scanning type electron microscope in which means are provided to periodically vary the electron beam intensity for thereby eliminating the above defects encountered with the prior art electron microscope of this type.

Another object of the present invention is to provide a scanning type electron microscope in which a pulsed electron beam of a certain waveform, for example, a pulsed electron beam of rectangular waveform is espe- 3 cially employed to facilitate the regulation of the electron beam current.

A further object of the present invention is to provide a scanning type electron microscope in which means are provided to make it possible to apply a D.C. bias voltage to a specimen so as to make it possible to measure the electron voltaic effect.

A still further object of the present invention is to provide a scanning electron microscope in which a tuned amplifier is employed in lieu of the D.C. amplifier described above so as to completely eliminate all the noises other than the modulating frequency for the electron beam, especially the secondary D.C. noise produced by the thermo-electromotive force or the like as pointed out previously.

Another object of the present invention is to provide a scanning type electron microscope having an improved resolution by virtue of the fact that its detection sensitivity is improved, its electron beam current is decreased, and the diameter of the electron beam is made far smaller than heretofore.

Still another object of the present invention is to provide a scanning type electron microscope in which an A.C. electrical signal can be solely detected by detecting means such as a photomultiplier so that the microscope examination may not be adversely affected even with a slight entrance of external light into the photomultiplier.

Yet another object of the present invention is to provide a scanning type electron microscope having a cathoderay oscilloscope whose intensity modulation can be effected according to an A.C. modulating system so as to simplify the structure of the modulation circuit and to reduce the cost of making such a circuit.

In order to attain the various objects as described above, the present invention provides a scanning type electron microscope which is characterized by the provision of means for emitting an electron beam, means for periodically varying the intensity of the emitted electron beam, means for scanning a specimen with the electron beam whose intensity is periodically varied, means for detecting the reflected electrons or the secondary electrons arriving from the specimen, means for taking out an A.C. component from the output of said detecting means, means for producing a scanning image in a synchronous relation with the scanning of said electron beam from the A.C. component derived in accordance with the scanned points of the specimen, and means for applying a bias voltage to the specimen.

While various practical advantages are derivable from the present invention, the present invention has especially notable advantages as those enumerated below.

(1) The electron microscope is especially effective for use in the examination of a specimen such as a semiconductor specimen by virtue of the fact that the scanning image can be freely observed while continuously applying a D.C. bias voltage to the specimen.

(2) Noises at frequencies other than the pulse frequency of electron beam modulating pulses, especially the secondary D.C. noise due to a thermo-electromotive force or the like, can be completely eliminated by virtue of the provision of means, for example, such as a tuned amplifier.

(3) The capability of complete elimination of the noises can improve the sensitivity of detection and can therefore reduce the electron beam current. As a result, it is possible to employ an electron beam of a smaller diameter than heretofore for thereby enhancing the resolution, and to reduce errors frequently involved in the measurement of the resistivity of a specimen.

(4) The prior requirement for completely shielding an electron detecting means such as a photomultiplier against any external light is utterly obviated. In the invention, the photomultiplier would not be adversely affected by a slight amount of external light incident thereupon since a signal to be detected has a pulsed waveform.

(5) The intensity modulation for the oscilloscope can be effected not only by a D.C. modulating system but also by an A.C. modulating system, which remarkably simplifies the structure of the intensity modulation system and renders the system quite inexpensive.

In the drawings:

FIG. 1 is a block diagram showing the structure of the prior art scanning type electron microscope described previously;

FIG. 2 is a block diagram showing the structure of the scanning type electron microscope embodying the present invention; and

FIG. 3 is a graphic illustration of waveforms to show the manner of operation of the electron microscope according to the present invention.

Referring to FIG. 2 in which like reference numerals are used to denote like parts appearing in FIG. 1, the scanning type electron microscope embodying the present invention includes an electron gun formed of an electron beam emitting filament 1, a Wehnelt cylinder 2 and an anode 3, an electron beam condenser lens 4, an electron beam deflecting device 5 for defiectingly directing an electron beam 16 to a specimen 6, a detector 7 such as a photomultiplier or a secondary-electron multiplier for detecting back-scattered electrons and secondary electrons, on-off switches 9, 10, 14, 27 and 28, a cathode-ray oscilloscope 11, a high voltage source 13 for the electron beam acceleration, and a bias voltage source 15. In the electron miscroscope, a transformer 17 is connected to the output of the detector 7 to serve as a circuit for cutting off a D.C. component from a signal detected by the detector 7. However, the transformer 17 may be replaced by a condenser. The electron miscroscope further includes therein a tuned amplifier 18, a phase-sensitive demodulator 19, a pulse generator 20, a phase shifter 21, a pulse transformer 22, an auxiliary bias voltage source 23, a D.C. amplifier 24, and a voltmeter 25. The transformer 17, the tuned amplifier 18, the phase-sensitive demodulator 19 and the phase shifter 21 constitute part of the detecting means, but it should be understood that any other suitable components may be employed in lieu thereof. It is to be understood further that the oscilloscope 11 in the electron miscroscope is internally equipped with a deflecting voltage source 12 as shown in FIG. 1.

As described previously, the scanning type electron microscope according to the present invention is adapted to operate with an electron beam whose intensity is periodically varied. To this end, pulses of a rectangular waveform at a pulse repetition frequency of, for example, 1 kilocycle per second are generated by the pulse generator 20 and are applied through the pulse transformer 22 to the Wehnelt cylinder 2. Thus, a pulsed electron beam 16 is emitted from the filament 1, but it should be understood that the pulse repetition frequency is in no way limited to the above-specified frequency of 1 kilocyle per second and any suitable frequency may be arbitrarily selected in lieu thereof. Further, the pulsed electron beam may not necessarily have the rectangular waveform as specified above. In case, however, the pulsed electron beam takes the form of a sine wave, it is considerably difficult to always obtain a waveform as shown in FIG. 3a which is completely free from any D.C. component, because of the drift variation or the like normally occurring in the output of the pulse generator 20, and a D.C. component B commonly unavoidably appears in a superposed relation as shown in FIG. 3b. The presence of such a D.C. component E inevitably results in the generation of a noise due to the thermoelectromotive force as pointed out previously. In order to cancel out the D.C. component accompanying the sine wave, a D.C. bias voltage of opposite polarity must be provided and this bias voltage must be varied with relation to the variation in the sine wave voltage. In this respect, a rectangular waveform as shown in FIG. 30 is preferred since the D.C. component as described above can be easily cancelled by merely suitably adjusting the bias voltage according to the variation in the peak value E of the rectangular Waveform.

The pulsed electron beam 16 is then deflected in a twodimensional fashion by the deflecting device 5 in a synchronous relation with the scanning on the oscilloscope 11 to thereby scan the specimen 6. The deflecting device 5 is designed to operate as an electrostatic type of deflecting system according to which the sweep signal for the oscilloscope 11 is applied to electrostatic deflecting plates 5' after being amplified by the D.C. amplifier 24. It will be understood, however, that the deflecting device 5 may be designed to operate as an electromagnetic type of deflecting system employing therein an electromagnetic deflecting coil.

The scanning of the specimen 6 may be done after turning the switches 9 and 27 on and urging the switch 14' to its grounded position. In this case, reflected electrons and secondary electrons arriving from the specimen 6 can be detected by the detector 7. Alternatively, the switches 9 and 27 may be turned off and the switch 10 may be turned on to derive a specimen current or an electron current flowing into the specimen 6, or the switches 10 and 27 may be turned on and the switch 14' may be urged to be connected with the bias voltage source 15 to thereby derive a voltage induced by the electron voltaic eflect.

Suppose, for example, that the specimen 6 is a semiconductor device having a p-n junction. Then, a pulsed voltage is induced due to the electron voltaic effect when the junction is scanned with the pulsed electron beam 16, and this pulsed voltage is supplied through the pulse transformer 17 into the tuned amplifier 18 for amplifying solely the pulse component at the pulse frequency of 1 kilocycle per second. The voltage induced in this case has a pulsating waveform whose amplitude is variable due to the fact that the D.C. bias component is superposed thereon. Since, however, the tuned amplifier 18 has the tuning circuit at the input side thereof which is operative to solely permit passage of an A.C. component therethrough, such A.C. component is amplified in the amplifier 18 and is then subjected to a synchronous rectification by the phase-sensitive demodulator 19 so as to be delivered therefrom as a pulsating signal. The pulses from the pulse generator 20 used for the modulation of the electron beam are utilized to provide a signal for the synchronous rectification in such a manner that a sinusoidal component is solely derived from the pulses and is supplied through the phase shifter 21 to the phasesensitive demodulator 19.

In the above arrangement, the D.C. bias voltage applied to the specimen 6 can be completely eliminated at the detecting side, and since the induced voltage is subjected to synchronous rectification by the sinusoidal component in the beam modulating pulses, a signal of the same polarity as the voltage induced at the junction can be detected. Therefore, a scanning image can be obtained on the oscilloscope 11 and the desired purpose can be attained when the switch 28 is closed and the pulsating signal having been subjected to the synchronous rectification is used to effect the intensity modulation of the oscilloscope 11. The intensity modulation system for the oscilloscope 11 is not of the prior D.C. modulation type as, for example, disclosed in Japan Applied Physics, No. 36, 1965, p. 1476, but of an A.C. modulation type as, for example, disclosed in The Cathode-Ray Tube and Its Applications, 1959, p. 222, by G. Parr and O. H. Davie, published by Reinhold Publishing Corporation, New York, because of the fact that the signal having been subjected to the synchronous rectification has a pulsating waveform having a frequency of 1 kilocycle per second. Thus, a universal oscilloscope priorly commonly employed in the art is sufficiently usable for the purpose.

The above description has referred to the formation of a scanning image from a voltage induced in the specimen due to the electron voltaic effect. Similarly, in the case of the detection of a specimen current or an electron current flowing into the specimen, the operation may be such that an A.C. component may solely be detected from the specimen current and may then be amplified. By this way of operation, noises other than the signal at the pulse frequency of 1 kilocycle per second can completely be eliminated and the detection sensitivity can remarkably be improved. Similarly, in the case of the detection of reflected electrons and secondary electrons, a circuitry of the same structure as that described above may be used in cooperation with the detector 7. which may be a photomultiplier, because its output is an A.C. signal. An alternative arrangement may be employed in which, without making the synchronous rectification and other operations, the detected A.C. signal may merely be rectified to give a D.C. signal and a D.C. modulation system may be employed to effect the intensity modulation of the oscilloscope 11. A change or modification may be suitably made in the operating system to suit the desired purpose.

In connection with the above case, the diffusion potential at a p-n junction, for example, may be cancelled out by a forward bias voltage in order to zeroize the voltage induced due to the electron voltaic effect. By so doing, it is possible to measure the diffusion potential on the basis of the value of such bias voltage. To this end, a D.C. bias voltage may be applied from the D.C. bias voltage source 15 to the semiconductor specimen 6 to provide the above-described forward bias voltage which may be read on the voltmeter 25. When, at the same time, the scanning image on the oscilloscope 11 is observed until the result of the observation on the scanning image indicates that the induced voltage is rendered zero, the reading of the voltmeter 25 at that moment indicates the value of the diffusion potential and thus the diffusion potential can easily be measured.

Further, in the case of such semiconductor specimen, its resistivity can also be measured by such an arrangement that the induced voltage is solely detected without application of the above-described D.C. bias voltage. As is commonly known, it is very important for the improvement in the properties and uniformity of a device such as a transistor made from a semiconductor material to know the resistivity of and its distribution in the semi conductor material.

The induced voltage V appearing across the specimen 6 due to the electron voltaic effect when the electron beam scans the specimen in the direction of its X-axis in the absence of any D.C. bias voltage is expressed by an equation,

where, I is the electron beam current, r is the resistance between the point of beam bombardment and a grounded point, and AV is the electromotive force developed by electrons and holes produced in the semiconductor specimen by the beam bombardment aud is proportional to the resistivity gradient a /e in the semiconductor specimen. Therefore, the relation VAV can be obtained if I alone can be made sufficiently small without decreasing the radiating energy of the electron beam. It will thus be understood that a value quite proportional to the resistivity p can be obtained by integrating the above value of V and the resistivity of the semiconductor specimen can thereby be easily measured.

In accordance with the present invention, the electron beam is pulsed as described previously and therefore any adverse effect of the thermo-electromotive force arising from the beam bombardment is substantially completely eliminated. Accordingly, an integrator 26 may be provided on the output side of the phase-sensitive demodulator 19, the switch 28 may be urged to its open position, and a switch 29 may be closed to supply the output of the integrator 26 to the oscilloscope 11 for the intensity modulation of the oscilloscope 11. Through the above manner of operation, the distribution of the 7 resistivity in the semiconductor specimen 6 can be observed in a two-dimensional fashion.

What is claimed is:

1. A scanning type electron microscope comprising electron beam generating means for emitting an electron beam, means for periodically varying the intensity of the emitted electron beam, means for scanning a specimen with the electron beam whose intensity is periodically varied, detecting means for detecting an electric signal originated in the specimen by the scanning with said electron beam, said electric signal including unwanted D.C. components and an A.C. component, said A.C. component containing information indicative of the characteristics of the specimen, filter means for passing only an A.C. component from the output of said detecting means, said filter means including phase-sensitive demodulator means for synchronously rectifying said A.C. information signal, and means for producing a scanning image of the specimen in synchronous relationship with the scanning of the electron beam from the signal derived from said filter means in accordance with the scanned points of the specimen, whereby undesired D.C. noise signals are eliminated.

2. A scanning type electron microscope according to claim 1, which further comprises a bias voltage source and a first switching means for applying a bias voltage to the specimen.

3. A scanning type electron microscope according to claim 1, wherein said filter means further includes blocking means for blocking passage of a DO component from said electric signal, a tuning type amplifier for amplifying the output of said blocking means and for delivering an A.C. information signal to said phasesensitive demodulator means in amplified form, second switching means inserted between said detecting means and said blocking means and a third switching means inserted between said specimen and said blocking means.

4. A scanning type electron microscope according to claim 1, wherein an integrator is interposed between the output of said filter means and an input of said scanning image producing means.

5. A scanning type electron microscope according to claim 3 'wherein said means for periodically varying the intensity of the emitted electron beam includes a square wave generator connected to said electron beam generating means and said demodulator to provide synchronous rectification of said A.C. component.

References Cited UNITED STATES PATENTS 2,330,930 10/1943 Snyder 25049.5 2,348,030 5/1944 Snyder 25O49.5 3,315,157 4/1967 Watanase et a1. 32462 3,403,332 9/1968 Watanase et a1. 25049.5 3,319,071 5/1967 Werth et a1. 250-43.5

WILLIAM F. LINDQUIST, Primary Examiner C. E. CHURCH, Assistant Examiner 

